The work for the coming year is a continuation of efforts to determine the effects of Na efflux of unfueling giant axons from Myxicola of ATP. The component of Na efflux of special interest is that due to Na:Ca exchange. Before the effect of ATPi depletion on Cao-dependent Na efflux in Myxicola giant axons can be effectively analyzed, it will be necessary to know the effects of ATPi reduction on the other components of Na efflux as well. Unfueling giant axons of internal ATP will be accomplished by prolonged internal dialysis with ATP-free media. This procedure will be followed on axons with (Na)i and (K)o set at values optimal for Na pumping. The same procedure will be followed when axons have been treated with ouabain as this inhibitor is used to increase the resolution of Cao-sensitive components of Na efflux. Finally, (Na)i will be increased to 100 microns M where Cao-sensitive Na efflux occurs at a high rate in the presence of ATP. The effects of ATP removal and readmission on reversed Na:Ca exchange will then be assessed at various levels of internal and external (Ca ions) to determine the influence of ATPi on membrane-site affinities for Ca ions.